Important electrophysiological, pharmacological, and biochemical changes occur in myocardial cells during development of the heart, which obviously affect its functional properties. Young embryonic chick hearts have spontaneous slowly-rising, Na- and Ca-dependent, TTX-resistant action potentials. The number of fast Na channels and IK1 channels increases during development so that old embryonic chick hearts have a typical fast- rising Na-dependent action potential. Thus, the types and number of channels changes during development. Channel properties and kinetics may also change during development (long-lasting openings of Ca channels in young vs. old embryonic chick hearts). The identity and properties of the cardiac Ca channels which conduct inward current during the action potential will be examined at different stages of development. In these studies, whole-cell voltage clamp and patch clamp techniques will be used in single cells of embryonic chick and fetal rat hearts at different stages of development. In the adult heart, cAMP-dependent phosphorylation regulates the function of slow Ca channels. Phosphorylation by other kinases may also regulate channel function. For example, cGMP-, calmodulin- and phospholipid-dependent phosphorylation regulates the function of slow Ca channels. Phosphorylation by other kinases may also regulate channel function. The proposed experiments seek to determine the role of protein phosphorylation during development of the embryonic chick heart. Changes in cAMP and cGMP levels occur during development. Calmodulin levels, cGMP levels and various protein kinase activities will be measured at different stages of development. Regulation of sarcolemmal proteins by the various protein kinases will be examined to determine whether the pattern of phosphorylation changes during development, as has been shown for cAMP-dependent phosphorylation. The role of dephosphorylation will also be examined electrophysiologically and biochemically. Regulation of cation channels by G-proteins will also be evaluated. Biochemical and electrophysiological experiments will be correlated to give a better understanding of channel properties and function during development. These studies are important, since the response of the heart to pathological conditions (e.g., myocardial ischemia) and to cardioactive drugs is largely determined by the types and properties of channels present.